Stability of volcanic ash aggregates and break-up processes

Abstract

Numerical modeling of ash plume dispersal is an important tool for forecasting and mitigating potential hazards from volcanic ash erupted during explosive volcanism. Recent tephra dispersal models have been expanded to account for dynamic ash aggregation processes. However, there are very few studies on rates of disaggregation during transport. It follows that current models regard ash aggregation as irrevocable and may therefore overestimate aggregation-enhanced sedimentation. In this experimental study, we use industrial granulation techniques to artificially produce aggregates. We subject these to impact tests and evaluate their resistance to break-up processes. We find a dependence of aggregate stability on primary particle size distribution and solid particle binder concentration. We posit that our findings could be combined with eruption source parameters and implemented in future tephra dispersal models.Numerous investigations, using field [1][2][3][4][5] , numerical 6, 7 and experimental 8-12 approaches have extended our understanding of the generation of volcanic ash aggregates. The control of volcanic ash aggregation on ash plume dispersal has also been demonstrated by field studies [13][14][15] and is now a common component in numerical modeling of volcanic ash dispersal [16][17][18][19] . However, an understanding of aggregate preservation potential during transport and sedimentation processes is not yet fully understood. Ash is exposed to strongly variable transport conditions that may control aggregation rates (e.g. wind speed, temperature, humidity, acidity, glass content of the ash, particle-particle interaction rates). Nevertheless, the same factors can also control aggregate preservation potential. Disaggregation processes resulting from the elastic mechanical stresses associated with particle-particle interactions may occur both during transport (aggregate-aggregate or aggregate-particle) as well as during sedimentation (aggregate-substrate). There are two key controls on aggregate stability: (1) the properties of the ash particles that form aggregates (i.e. primary particle size, morphology and the aggregate binder agent [20][21][22] ) and (2) aggregate size, shape and roughness. Aggregates fail to remain intact and cohesive if extrinsic elastic stress is higher than the tensile strength of inter-particle contact areas. Analysis of large volcanic aggregates (i.e. mm-to cm-size) from several locations has shown secondary mineral phases like NaCl, MgSO 4 or CaSO 4 2, 8, 12, 23-25 that act as binding agents between particles . Crack initiation in such solid salt bridges may lead to either internal failure of the solid bridge (cohesive failure) or failure of the contact line between solid bridge and particle (adhesive failure 26 ). Depending on the initial impact energy, particles may be chipped off from the aggregate surface (low impact energy), the aggregate may fragment into several parts (moderate impact energy) or the aggregate may wholly disaggregate into primary particles (high impact ener...

Aggregation in particle rich environments: a textural study of examples from volcanic eruptions, meteorite impacts, and fluidized bed processing

“…The influence of the different starting materials in the particle plume seems to be of subordinate importance. Volcanic field studies and experiments have shown that preservation of aggregates is critically depending on the availability of soluble salt compounds such as NaCl or CaSO 4 (Gilbert and Lane 1994 ; Mueller et al 2017b ). Volcanic eruptions offer two ways to precipitate salts on ash surfaces: (1) through precipitation of salt crystals (e.g., NaCl) out of salt rich brines (e.g., interaction of volcanic ejecta with sea water during phreatomagmatic eruptions) or (2) through diffusion-driven precipitation after the chemical interaction of acid solutions (e.g., HCl or H 2 SO 4 ) with ash particles (e.g., Ayris et al 2013 , 2014 ).…”

“…Volcanic eruptions offer two ways to precipitate salts on ash surfaces: (1) through precipitation of salt crystals (e.g., NaCl) out of salt rich brines (e.g., interaction of volcanic ejecta with sea water during phreatomagmatic eruptions) or (2) through diffusion-driven precipitation after the chemical interaction of acid solutions (e.g., HCl or H 2 SO 4 ) with ash particles (e.g., Ayris et al 2013 , 2014 ). Whereas the latter mechanism may also apply for impact events, salts for binding aggregates here may further depend on the chemistry of the target material and hence significantly influence aggregation efficiency: carbonite rich cherts as in the Ries area may significantly boost the generation/presence of such salts like gypsum or calcium chloride on ejecta particles, which will in turn improve aggregate stability and their chance of preservation (Mueller et al 2017b ).…”

“…In both cases, the liquid phase induces aggregation following collisions, and the resultant salts cement the particles together after evaporation of the liquid. Salts (mainly NaCl) are either generated through re-crystallization upon H 2 O evaporation, or through chemical reactions between the HCl phase and the glass (Mueller et al 2017b ). The aggregates produced in this manner are between 0.5–5 mm in size, depending on experimental parameters (Mueller et al 2016 ).…”

Modeling Eruption Source Parameters by Integrating Field, Ground‐Based, and Satellite‐Based Measurements: The Case of the 23 February 2013 Etna Paroxysm

“…One of the main goals of modern volcanology is a better understanding and quantification of eruption source parameters (ESP) governing tephra dispersal during a volcanic crisis. This is done using field (e.g., Andronico, Cristaldi, et al, ; Andronico, Scollo, et al, ; Andronico, Scollo, Lo Castro, et al, ), remote‐sensing retrievals (e.g., Corradini et al, , ; Gouhier et al, ; Scollo et al, , ), laboratory experiments (e.g., Bagheri & Bonadonna, ; Cigala et al, ; Mueller, Ayris, et al, ; Mueller, Kueppers, et al, ), and numerical models (e.g., Bonadonna & Costa, ; Folch et al, ; Scollo et al, ). ESP assessment (e.g., Folch, ; Mastin et al, ) involves the estimation, among others, of the mass eruption rate (MER), which combined with the eruption duration provides the total erupted mass (TEM).…”

Formation of stable aggregates by fluid-assembled solid bridges

“…U nderstanding the stability and strength of particle assemblies subject to disruptive forces is essential for predicting the macroscopic mechanical behavior of geological, biological, and industrial particulate materials (1)(2)(3). Hallmarks of such systems include size polydispersity and nonequilibrium dynamics (4,5).…”